1. Field of the Invention
The present invention relates to a sheet transport apparatus for transporting sheets and an image forming apparatus having the sheet transport apparatus.
2. Description of the Related Art
Known image forming apparatuses for forming images on a sheet are provided with a sheet transport apparatus for transporting sheets. Examples of image forming apparatuses include copiers, printers, facsimiles, and multifunction machines combining the functions of copiers, printers, and facsimiles.
Some sheet transport apparatuses detect the thickness of a sheet. Such sheet transport apparatuses are provided with a sheet detector that detects, for example, a position where, in the image forming apparatus, a sheet is currently being transported and the thickness of the sheet.
For example, a sheet detector included in an electrophotographic-type copier detects the movement of a sheet fed from the sheet cassette, and allows the detected timing to be used as information for controlling the sheet transport apparatus and image forming section on the downstream side.
Known sheet detectors will be described below.
Examples of Known Sheet Detector Detecting Arrival of Sheet
<First Sheet Detector>
A first example of a sheet detector detecting the arrival of a sheet is a photointerrupter sensor 258 shown in FIG. 10. The photointerrupter sensor 258 includes a rotatable flag 251 arranged in a position that blocks the sheet path, and a photointerrupter 253 detecting that detection light 253a is intercepted. A spring 252a presses the flag 251 into contact with a stopper 252b. 
When a sheet S is brought into contact with the flag 251 in the photointerrupter sensor 258, the flag 251 is rotated about the rotation shaft 251a and causes a light-shielding section 251b to block the detection light 253a . When the detection light 253a is blocked, the photointerrupter sensor 258 emits an electronic signal based on the determination that the sheet S has arrived. The electronic signal is transmitted to a controller (not shown) that controls the entire image forming apparatus.
<Second Sheet Detector>
A second example of a sheet detector is a light transmission sensor 260 shown in FIG. 11. The light transmission sensor 260 detects a sheet when the sheet blocks the optical axis. The light transmission sensor 260 includes a light emitter 260a emitting detection light 260c and a light receptor 260b. Unlike the photointerrupter sensor 258 in FIG. 10, the light transmission sensor 260 has no flag blocking the sheet path. This is advantageous in that the front edge of the sheet is not damaged even if the sheet is thin.
Examples of Known Sheet Detector Detecting Arrival and Thickness of Sheet
Electrophotographic-type image forming apparatuses often detect not only the movement of a sheet, but also detect the thickness of a sheet to control the operation of the image forming section. For example, in an electrophotographic-type image forming apparatus, which uses electric power to transfer toner to a sheet, it is desired that a voltage applied to the sheet be adjusted according to the thickness of the sheet.
The thickness information is also used to control the sheet transport mechanism. Before enabling the sheet transport mechanism to feed a sheet to the image forming section, the image forming apparatus brings the front edge of the sheet into contact with a resist roller at rest to correct the skew of the sheet, adjusts timing for starting the rotation of the resist roller, thereby adjusting timing for feeding the sheet to the image forming section. After bringing the sheet into contact with the resist roller, the image forming apparatus causes a transport roller, which allows a sheet to be fed into the resist roller, to rotate for a predetermined time (t) to create a loop in front of the resist roller. The force of the loop causes the front edge of the sheet to be reliably pressed against the resist roller, thereby allowing the skew of the sheet to be corrected. The time (t) is determined according to the thickness of the sheet. For example, for a thin sheet, the time (t) must be long enough to ensure the pressing force with which the sheet is pressed against the resist roller.
Since it is often required for image forming apparatuses to detect the thickness of the sheet, the following sheet detectors are proposed.
<Third Sheet Detector>
Referring to FIG. 12, a sheet detector 281 combines a sheet transport mechanism 282 and a contact-type probe sensor 264. The sheet transport mechanism 282 is configured such that a sheet S is introduced into the nip point between a roller 262a attached to a roller shaft 263a that is vertically displaceable, and a roller 262b attached to a roller shaft 263b that is secured so as not to be vertically displaced. The sheet detector 281 uses the contact-type probe sensor 264 to measure the displacement of the roller shaft 263a, the displacement being associated with the passage of the sheet S. This not only allows the detection of the arrival of the sheet S, but also allows the detection of the thickness of the sheet S. This configuration is disclosed in Japanese Patent Laid-Open No. 07-215538.
<Fourth Sheet Detector>
Similar to the sheet detector 281 in FIG. 12, a sheet detector 283 in FIG. 13 measures the displacement of a roller 271 to detect the arrival and thickness of a sheet. The sheet detector 283 differs from the sheet detector 281 in that it has a sheet thickness sensor 270 using reflecting light 270a to measure the displacement. The sheet detector 283 controls a transfer charging device 274 according to the thickness and electric resistance value of the sheet. The transfer charging device 274 transfers toner images on a photoconductive drum (not shown) onto the sheet. This configuration is disclosed in Japanese Patent Laid-Open No. 05-313516.
There is another proposed method to detect the displacement of a roller. In this method, a pressure sensor supported by an elastic member is pressed against a roller shaft, and a change in pressure is interpreted as the displacement of the roller. However, this method has problems in that the pressure sensor cannot easily detect the arrival of a thin sheet unless the spring constant of the elastic member is high enough, and that the nip pressure of the roller becomes unstable if the spring constant is too high.
The above-described sheet detectors that are proposed or already in practical use have problems described in the following (1) to (5). For example, known sheet detectors with such problems cannot easily transport a thin sheet, cannot be installed in a desired location, have a low accuracy in detecting the position or thickness of a sheet, and malfunction in the detection of the position or thickness of a sheet. Moreover, known image forming apparatuses having a sheet detector with these problems have a low accuracy in forming images on a sheet.
(1) The photointerrupter sensor 258 in FIG. 10 may obstruct the transport of a thin sheet.
(2) Problems in installation space: In the photointerrupter sensor 258, the rotation shaft 251a of the flag 251 and the photointerrupter 253 must be placed close to the sheet paths. Business machines, which are typically required to be small in size, have many sections where a plurality of connected and crossed sheet paths are densely arranged. Such a section may not be able to provide enough space to accommodate the photointerrupter 253. Similarly, installation space for the light transmission sensor 260 in FIG. 11, the contact-type probe sensor 264 in FIG. 12, and the sheet thickness sensor 270 in FIG. 13 may not be large enough.
(3) Problems in Installation: Since it is required for the photointerrupter sensor 258 that the positional relationship between the rotation shaft 251a and the photointerrupter 253 be kept constant, their attaching parts must be stable. If the rotation shaft 251a and the photointerrupter 253 need to be attached to different members, instability of the attaching parts affects detection accuracy. The same applies to the light transmission sensor 260 in FIG. 11.
For the contact-type probe sensor 264 in FIG. 12 and the sheet thickness sensor 270 in FIG. 13, an unstable positional relationship with respect to the respective rollers may cause detection errors. That is, instability of a base to which the sensor is attached causes detection errors.
(4) Problems of Dirt on Sensor: The light transmission sensor 260 in FIG. 11 and the sheet thickness sensor 270 in FIG. 13 may malfunction if the emitter or receiver of the detection light is soiled with paper dust from the sheet, abrasion dust and oil from the drive mechanisms, and the like.
(5) Problems of External Vibrations: The contact-type probe sensor 264 in FIG. 12 and the sheet thickness sensor 270 in FIG. 13 may malfunction if the roller 262a or the roller 271 is displaced due to vibrations transmitted from outside the sheet transport apparatus or generated inside the sheet transport apparatus. Detection errors can be prevented, to some extent, if the frame of the sheet transport apparatus is provided with an acceleration sensor such that the amount of displacement of the roller can be compared to the acceleration detected by the acceleration sensor. However, since acceleration applied to the frame and the amount of displacement of the roller are different types of physical quantities, it is difficult to completely prevent detection errors even if some predictions can be made about the relationship between the acceleration and the amount of displacement. The same applies to the case where a pressure sensor is used to detect the displacement of the roller.